Showing papers in "Reviews of Geophysics in 2001"

The quasi-biennial oscillation

Abstract: The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (∼16–50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes. The effects of the QBO are not confined to atmospheric dynamics. Chemical constituents, such as ozone, water vapor, and methane, are affected by circulation changes induced by the QBO. There are also substantial QBO signals in many of the shorter-lived chemical constituents. Through modulation of extratropical wave propagation, the QBO has an effect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratosphere affects surface weather patterns, providing a mechanism for the QBO to have an effect at the Earth's surface. As more data sources (e.g., wind and temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. This review covers the current state of knowledge of the tropical QBO, its extratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (∼0–16 km) and mesosphere (∼50–100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the field, as well as a source of information and references for specialists. The history of research on the QBO is discussed only briefly, and the reader is referred to several historical review papers. The basic theory of the QBO is summarized, and tutorial references are provided.

Scaling of fracture systems in geological media

Abstract: Scaling in fracture systems has become an active field of research in the last 25 years motivated by practical applications in hazardous waste disposal, hy- drocarbon reservoir management, and earthquake haz- ard assessment. Relevant publications are therefore spread widely through the literature. Although it is rec- ognized that some fracture systems are best described by scale-limited laws (lognormal, exponential), it is now recognized that power laws and fractal geometry provide widely applicable descriptive tools for fracture system characterization. A key argument for power law and fractal scaling is the absence of characteristic length scales in the fracture growth process. All power law and fractal characteristics in nature must have upper and lower bounds. This topic has been largely neglected, but recent studies emphasize the importance of layering on all scales in limiting the scaling characteristics of natural fracture systems. The determination of power law expo- nents and fractal dimensions from observations, al- though outwardly simple, is problematic, and uncritical use of analysis techniques has resulted in inaccurate and even meaningless exponents. We review these tech- niques and suggest guidelines for the accurate and ob- jective estimation of exponents and fractal dimensions. Syntheses of length, displacement, aperture power law exponents, and fractal dimensions are found, after crit- ical appraisal of published studies, to show a wide vari- ation, frequently spanning the theoretically possible range. Extrapolations from one dimension to two and from two dimensions to three are found to be nontrivial, and simple laws must be used with caution. Directions for future research include improved techniques for gathering data sets over great scale ranges and more rigorous application of existing analysis methods. More data are needed on joints and veins to illuminate the differences between different fracture modes. The phys- ical causes of power law scaling and variation in expo- nents and fractal dimensions are still poorly understood.

Influence of the spatial distribution of vegetation and soils on the prediction of cumulus Convective rainfall

Abstract: This paper uses published work to demonstrate the link between surface moisture and heat fluxes and cumulus convective rainfall. The Earth's surface role with respect to the surface energy and moisture budgets is examined. Changes in land-surface properties are shown to influence the heat and moisture fluxes within the planetary boundary layer, convective available potential energy, and other measures of the deep cumulus cloud activity. The spatial structure of the surface heating, as influenced by landscape patterning, produces focused regions for deep cumulonimbus convection. In the tropics, and during midlatitude summers, deep cumulus convection has apparently been significantly altered as a result of landscape changes. These alterations in cumulus convection teleconnect to higher latitudes, which significantly alters the weather in those regions. The effect of tropical deforestation is most clearly defined in the winter hemisphere. In the context of climate, landscape processes are shown to be as much a part of the climate system as are atmospheric processes.

Stagnant slabs in the upper and lower mantle transition region

Abstract: We made a region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660-km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre-Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

Ozone trends: A review

Abstract: Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long-term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine- and bromine-containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone-depleting substances is the “Antarctic ozone hole.” Long-term observations indicate that stratospheric ozone depletion in the southern winter-spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground-based measurements show no significant trends in the tropics but significant long-term decreasing trends in the northern and southern midlatitudes (of the order of 2–4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by −7.4±2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, −5.1±1.8% at 20 km and −7.3±4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone-depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.

Snow on Antarctic sea ice

Abstract: Snow on Antarctic sea ice plays a complex and highly variable role in air-sea-ice interaction processes and the global climate system. This paper presents snow data collected during the past ten years, and reviews major findings. These include: differences in regional and seasonal snow properties and thicknesses; the unique consequences of snow on Antarctic pack ice relative to the Arctic (e.g. the importance of flooding and snow-ice formation); the potential impact if global change increases snowfall; lower observed values of snow thermal conductivity than those used in models; periodic large-scale melt in winter; and the contrast in summer melt in the Antarctic and Arctic. The new findings have significant implications for modelling and remote-sensing studies. Different snow properties from Arctic conditions are recommended for use in Antarctic models; similar differences could affect the interpretation of remote-sensing data over sea ice.

Measuring snow and glacier ice properties from satellite

Abstract: Satellite remote sensing is a convenient tool for studying snow and glacier ice, allowing us to conduct research over large and otherwise inaccessible areas. This paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing. These methods have been improving with the use of new satellite sensors, like the synthetic aperture radar (SAR) during the last decade, leading to the development of new and powerful methods, such as SAR interferometry for glacier velocity, digital elevation model generation of ice sheets, or snow cover mapping. Some methods still try to overcome the limitations of present sensors, but future satellites will have much increased capability, for example, the ability to measure the whole optical spectrum or SAR sensors with multiple polarization or frequencies. Among the methods presented are the satellite-derived determination of surface albedo, snow extent, snow volume, snow grain size, surface temperature, glacier facies, glacier velocities, glacier extent, and ice sheet topography. In this review, emphasis is put on the principles and theory of each satellite remote sensing method. An extensive list of references, with an emphasis on studies from the 1990s, allows the reader to delve into specific topics.

High-pressure elastic properties of major materials of Earth's mantle from first principles

Abstract: The elasticity of materials is important for our understanding of processes ranging from brittle failure, to flexure, to the propagation of elastic waves. Seismologically revealed structure of the Earth's mantle, including the radial (one-dimensional) profile, lateral heterogeneity, and anisotropy are determined largely by the elasticity of the materials that make up this region. Despite its importance to geophysics, our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the Earth's mantle is still limited: Measuring the elastic constants at elevated pressure-temperature conditions in the laboratory remains a major challenge. Over the past several years, another approach has been developed based on first-principles quantum mechanical theory. First-principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. A review of high-pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated Mg and Ca silicates and Mg, Ca, Al, and Si oxides. This is particularly apparent in the acoustic anisotropy, which is essential for understanding the relationship between seismically observed anisotropy and mantle flow. The acoustic anisotropy of the phases studied varies from zero to more than 50% and is found to depend on pressure strongly, and in some cases nonmonotonically. For example, the anisotropy in MgO decreases with pressure up to 15 GPa before increasing upon further compression, reaching 50% at a pressure of 130 GPa. Compression also has a strong effect on the elasticity through pressure-induced phase transitions in several systems. For example, the transition from stishovite to CaCl2 structure in silica is accompanied by a discontinuous change in the shear (S) wave velocity that is so large (60%) that it may be observable seismologically. Unifying patterns emerge as well: Eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases. This is in contrast to an evaluation of Birch's law, which shows that this systematic accounts only roughly for the effect of pressure, composition, and structure on the longitudinal (P) wave velocity. The growing body of theoretical work now allows a detailed comparison with seismological observations. The athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the P and S wave velocities of the phases studied. An examination of future directions focuses on strategies for extending first-principles studies to more challenging but geophysically relevant situations such as solid solutions, high-temperature conditions, and mineral composites.

Glacial cycles: Toward a new paradigm

Abstract: The largest environmental changes in the recent geological history of the Earth are undoubtedly the successions of glacial and interglacial times. It has been clearly demonstrated that changes in the orbital parameters of our planet have a crucial role in these cycles. Nevertheless, several problems in classical astro- nomical theory of paleoclimate have indeed been iden- tified: (1) The main cyclicity in the paleoclimatic record is close to 100,000 years, but there is no significant orbitally induced changes in the radiative forcing of the Earth in this frequency range (the "100-kyr problem"); (2) the most prominent glacial-interglacial transition occurs at a time of minimal orbital variations (the "stage 11 problem); and (3) at ;0.8 Ma a change from a 41-kyr dominant periodicity to a 100-kyr periodicity occurred without major changes in orbital forcing or in the Earth's configuration (the "late Pleistocene transition prob- lem"). Additionally, the traditional view states that the climate system changes slowly and continuously together with the slow evolution of the large continental ice sheets, whereas recent high-resolution data from ice and marine sediment cores do not support such a gradual scenario. Most of the temperature rise at the last termi- nation occurred over a few decades in the Northern Hemisphere, indicating a major and abrupt reorganiza- tion of the ocean-atmosphere system. Similarly, huge iceberg discharges during glacial times, known as Hein- rich events, clearly demonstrate that ice sheet changes may also be sometimes quite abrupt. In light of these recent paleoclimatic data the Earth climate system ap- pears much more unstable and seems to jump abruptly between different quasi steady states. Using the concept of thresholds, this new paradigm can be easily integrated into classical astronomical theory and compared with recent observational evidence. If the ice sheet changes are, by definition, the central phenomenon of glacial- interglacial cycles, other components of the climate sys- tem (atmospheric CO2 concentration, Southern Ocean productivity, or global deep-ocean circulation) may play an even more fundamental role in these climatic cycles.

The dynamics of ocean heat transport variability

Abstract: The north-south heat transport is the prime manifestation of the ocean's role in global climate, but understanding of its variability has been fragmentary owing to uncertainties in observational analyses, limitations in models, and the lack of a convincing mechanism. We review the dynamics of global ocean heat transport variability, with an emphasis on timescales from monthly to interannual. We synthesize relatively simple dynamical ideas and show that together they explain heat transport variability in a state-of-the-art, high-resolution ocean general circulation model. Globally, the cross-equatorial seasonal heat transport fluctuations are close to ±3 × 1015 W, the same amplitude as the cross-equatorial seasonal atmospheric energy transport. The variability is concentrated within 20° of the equator and dominated by the annual cycle. The majority of the variability is due to wind-induced current fluctuations in which the time-varying wind drives Ekman layer mass transports that are compensated by depth-independent return flows. The temperature difference between the mass transports gives rise to the time-dependent heat transport. It is found that in the heat budget the divergence of the time-varying heat transport is largely balanced by changes in heat storage. Despite the Ekman transport's strong impact on the time-dependent heat transport, the largely depth-independent character of its associated meridional overturning stream function means that it does not affect estimates of the time-mean heat transport made by one-time hydrographic surveys. Away from the tropics the heat transport variability associated with the depth-independent gyre and depth-dependent circulations is much weaker than the Ekman variability. The non-Ekman contributions can amount to a 0.2–0.4 × 1015 W standard deviation in the heat transport estimated from a one-time hydrographic survey.

Present and past nonanthropogenic CO2 degassing from the solid earth

Abstract: Global carbon cycle models suggest that CO2 degassing from the solid Earth has been a primary control of paleoatmospheric CO2 contents and through the greenhouse effect, of global paleotemperatures. Because such models utilize simplified and indirect assumptions about CO2 degassing, improved quantification is warranted. Present-day CO2 degassing provides a baseline for modeling the global carbon cycle and provides insight into the geologic regimes of paleodegassing. Mid-ocean ridges (MORs) discharge 1–3 × 1012 mol/yr of CO2 and consume ∼3.5 × 1012 mol/yr of CO2 by carbonate formation in MOR hydrothermal systems. Excluding MORs as a net source of CO2 to the atmosphere, the total CO2 discharge from subaerial volcanism is estimated at ∼2.0–2.5 × 1012 mol/yr. Because this flux is lower than estimates of the global consumption of atmospheric CO2 by subaerial silicate weathering, other CO2 sources are required to balance the global carbon cycle. Nonvolcanic CO2 degassing (i.e., emission not from the craters or flanks of volcanos), which is prevalent in high heat flow regimes that are primarily located at plate boundaries, could contribute the additional CO2 that is apparently necessary to balance the global carbon cycle. Oxidation of methane emitted from serpentinization of ultramafics and from thermocatalysis of organic matter provides an additional, albeit unquantified, source of CO2 to the atmosphere. Magmatic CO2 degassing was probably a major contributor to global warming during the Cretaceous. Metamorphic CO2 degassing from regimes of shallow, pluton-related low-pressure regional metamorphism may have significantly contributed to global warming during the Cretaceous and Paleocene/Eocene. CO2 degassing associated with continental rifting of Pangaea may have contributed to the global warming that was initiated in the Jurassic. During the Cretaceous, global warming initiated by CO2 degassing of flood basalts, and consequent rapid release of large quantities of CH4 by decomposition of gas hydrates (clathrates), could have caused widespread extinctions of organisms.

Using chemical tracers to assess ocean models

Abstract: Chemical tracers can be used to assess the simulated circulation in ocean models. Tracers that have been used in this context include tritium, chlorofluorocarbons, natural and bomb-produced radiocarbon, and to a lesser extent, oxygen, silicate, phosphate, isotopes of organic and inorganic carbon compounds, and certain noble gases (e.g., helium and argon). This paper reviews the use of chemical tracers in assessing the circulation and flow patterns in global and regional ocean models. It will be shown that crucial information can be derived from chemcial tracers that cannot be obtained from temperature-salinity (T-S) alone. In fact, it turns out that a model with a good representation of T-S can have significant errors in simulated circulation, so checking a model’s ability to capture chemical tracer patterns is vital. Natural chemical tracers such as isotopes of carbon, argon, and oxygen are useful for examining the model representation of old water masses, such as North Pacific and Circumpolar Deep Water. Anthropogenic or transient tracers, such as tritium, chlorofluorocarbons, and bomb-produced 14C, are best suited for analyzing model circulation over decadal timescales, such as thermocline ventilation, the renewal of Antarctic Intermediate Water, and the ventilation pathways of North Atlantic Deep Water and Antarctic Bottom Water. Tracer model studies have helped to reveal inadequacies in the model representation of certain water mass formation processes, for example, convection, downslope flows, and deep ocean currents. They show how coarse models can chronically exaggerate the spatial scales of open-ocean convection and deep currents while underestimating deep flow rates and diffusing downslope flows with excessive lateral mixing. Higher-resolution models typically only resolve thermocline ventilation because of shorter integration times, and most resort to high-latitude T-S restoring to simulate reasonable interior water mass characteristics. This can be seen to result in spuriously weak chemical tracer uptake at high latitudes due to suppressed convective overturn and vertical motion. Overall, the simulation of chemical tracers is strongly recommended in model assessment studies and as a tool for analyzing water mass mixing and transformation in ocean models. We argue that a cost-effective approach is to simulate natural radiocarbon to assess long-timescale processes, and CFCs for decadal to interdecadal ocean ventilation.

Current controversies in magnetospheric physics

Abstract: The scientific ascent of humankind comes in two forms: incremental progress with traditional thinking, and paradigm transition. The latter typically engenders intense debate, often long lasting, before the transition of the paradigm is deemed acceptable. The magnetospheric discipline is no exception, especially in view of the sparse observations available to reach conclusive theory-data closure. Scientific controversies serve an important role in identifying significant unresolved issues for progress to be made. This review highlights key issues on four controversies present in magnetospheric physics: (1) the on-and-off debate on whether the magnetic field combined with the plasma bulk flow or the electric field combined with the current density is the primary quantity in treating magnetospheric problems; (2) the proper interpretation of transient dayside magnetospheric phenomena, i.e., whether they are related to flux transfer events, plasma transfer events, or solar wind pressure pulses; (3) the physical processes responsible for substorm onset; and (4) justifications for substorms or enhanced magnetospheric convection as the cause of magnetic storms. In issue 1 the merits and limitations of the two approaches are expounded. In issue 2 the predicted similarities and differences between the three interpretations are summarized. In issue 3 the strengths and weaknesses of prominent existing substorm models are elaborated on. In issue 4 the two contributors to storms are recognized and combined. Each controversy has an element of a paradigm transition. The resolutions of these controversies appear to also have one common element in that the presumption of only one theory to be correct may not be valid; a synthesis of existing theories may provide a better understanding of all features associated with the phenomenon.

The role of the ionosphere in aurora and space weather

Abstract: Recent research strongly suggests that the ionosphere plays a crucial role in the dynamics of space weather. Although the ionosphere is by volume only a small fraction of the magnetosphere, it serves as a variably conducting boundary, modulating the global electrodynamic circuit in crucial ways. A striking example is the behavior of intense aurora, which have recently been discovered to occur only when the background ionospheric conductivity is low. It is now clear that auroral acceleration occurs at the interface between the ionosphere and the magnetosphere and is controlled by magnetospheric-ionospheric coupling, with the solar cycle variations arising from a surprising source: variations in solar EUV flux. The discovery of diverging electric fields with their possibly corresponding black aurora provides a new symmetry to magnetosphere-ionosphere coupling processes. The far-reaching scope of the ionosphere in space weather problems is illustrated here in several ways. Ionospheric convection is suggested to be a major player in space weather, by creating global coherence in the magnetosphere on timescales not otherwise practical. Even a problem seemingly as far removed as possible from the ionosphere, namely, that of charge neutrality in polar rain (superthermal solar wind electron) entry into the distant magnetotail, is shown to be coupled to the problem of polar wind outflow from the ionosphere.

Hydrology of Yucca Mountain, Nevada

Abstract: Yucca Mountain, located in southern Nevada in the Mojave Desert, is being considered as a geologic repository for high-level radioactive waste. Although the site is arid, previous studies indicate net infiltration rates of 5–10 mm yr−1 under current climate conditions. Unsaturated flow of water through the mountain generally is vertical and rapid through the fractures of the welded tuffs and slow through the matrix of the nonwelded tuffs. The vitric-zeolitic boundary of the nonwelded tuffs below the potential repository, where it exists, causes perching and substantial lateral flow that eventually flows through faults near the eastern edge of the potential repository and recharges the underlying groundwater system. Fast pathways are located where water flows relatively quickly through the unsaturated zone to the water table. For the bulk of the water a large part of the travel time from land surface to the potential repository horizon (∼300 m below land surface) is through the interlayered, low fracture density, nonwelded tuff where flow is predominately through the matrix. The unsaturated zone at Yucca Mountain is being modeled using a three-dimensional, dual-continuum numerical model to predict the results of measurements and observations in new boreholes and excavations. The interaction between experimentalists and modelers is providing confidence in the conceptual model and the numerical model and is providing researchers with the ability to plan further testing and to evaluate the usefulness or necessity of further data collection.

A review of critical ionization velocity

Abstract: This paper reviews the critical ionization velocity (CIV) phenomenon, with inclusion of recent research. CIV, suggested by Alfven in 1954 as part of a larger cosmological theory accounting for the formation of the solar system, is controversial in that laboratory and space experiments to confirm its validity have yielded conflicting results. Theoretical analysis has suggested that the driving mechanism for CIV is some form of beam-plasma instability, such as the lower hybrid instability, which leads to rapid energization of ambient electrons so that they gain enough energy to ionize the beam of neutral atoms by electron impact. The newly created beam ions energize the instability, thus fostering a cyclic process that may lead to an avalanche ionization. Because the implications of this process, if correct, are widespread, it has become important to establish a theoretical framework for its presence and its occurrence. This framework includes a variety of microscopic chemical and physicochemical processes, such as line excitation, formation of metastable states by electron impact, ionization of metastable states by electron impact, and ion-electron recombination in the case of molecular ions. These reactions may occur in CIV experiments both in the laboratory and in space. Numerical computer models have been able to not only simulate CIV but also reveal details in nonlinear plasma evolution together with electron impact ionization of the neutral particles. Theories, laboratory experiments, and computer simulations have all shown CIV as feasible and reasonably understood, although all CIV experiments in space have yielded negative results with perhaps three exceptions. In the CIV experiments in space, not only the ionization yields were low, but also non-CIV processes such as charge exchange, associative ionization, and stripping ionization may have occurred and may be easily mistaken as CIV. We also discuss the conditions under which the laboratory and the space experiments are carried out and highlight the differences, some of which may explain the different results.

Sources of light in the deep ocean

Abstract: Studies during recent decades have shown that the deep ocean (depths below where solar luminance plays a direct environmental role) is far from a dark, cold, lifeless region. Evidence obtained by utilizing a variety of photo-optical devices, providing spatial, temporal, and spectral information, has demonstrated that this portion of the Earth is a region rich in life and light. Findings to date have provided challenges for geologists, physicists, biologists, chemists, and oceanographers, and the sharing of techniques and expertise among these disciplines has demonstrated the rewards to be gained from interdisciplinary research. Bioluminescence has been found far below the depths at which it has received most attention historically. The study of this phenomenon is complicated by the fact that the measuring apparatus itself causes a stimulation of the luminescence so that a true background will be difficult to determine. Nuclear physics has played a role in that the electron resulting from the decay of an isotope of potassium (K40) provides an ubiquitous background of light through a process known as Cerenkov radiation. The possibility of light generated by cosmic rays must also be taken into account. An intriguing source of light at the very bottom of the sea is found at hydrothermal vents. Here are found not only light but also abundant life. At the vent orifice, temperatures are found to be as high as 250°–400°C. A large component of the light is due to thermal radiation. However, the light in the wavelengths 450–600 nm is significantly greater than the thermal flux at those wavelengths. Identifying the physical mechanisms that may account for this excess is important in providing insight into the processes occurring in the vents and plumes and in the accompanying ecosystem.